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Concept
Virtual address space
Summary
In computing, a virtual address space (VAS) or address space is the set of ranges of virtual addresses that an operating system makes available to a process. The range of virtual addresses usually starts at a low address and can extend to the highest address allowed by the computer's instruction set architecture and supported by the operating system's pointer size implementation, which can be 4 bytes for 32-bit or 8 bytes for 64-bit OS versions. This provides several benefits, one of which is security through process isolation assuming each process is given a separate address space.
In the following description, the terminology used will be particular to the Windows NT operating system, but the concepts are applicable to other virtual memory operating systems.
When a new application on a 32-bit OS is executed, the process has a 4 GiB VAS: each one of the memory addresses (from 0 to 232 − 1) in that space can have a single byte as a value. Initially, none of them have values ('-' represents no value). Using or setting values in such a VAS would cause a memory exception.
0 4 GiB
VAS |----------------------------------------------|
Then the application's executable file is mapped into the VAS. Addresses in the process VAS are mapped to bytes in the exe file. The OS manages the mapping:
0 4 GiB
VAS |---vvv----------------------------------------|
mapping |||
file bytes app
The v's are values from bytes in the . Then, required DLL files are mapped (this includes custom libraries as well as system ones such as kernel32.dll and user32.dll):
0 4 GiB
VAS |---vvv--------vvvvvv---vvvv-------------------|
mapping ||| |||||| ||||
file bytes app kernel user
The process then starts executing bytes in the EXE file. However, the only way the process can use or set '-' values in its VAS is to ask the OS to map them to bytes from a file. A common way to use VAS memory in this way is to map it to the .
Official source
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